For organic electroluminescent devices to become fully competitive with their inorganic counterparts, it is desirable that their power conversion efficiencies be increased at competitive costs. The power conversion efficiency is defined as the ratio of power output to power input, usually watt per watt, and is a function of the driving voltage of the device. For driving voltages that utilize economical drive circuitry, that is, voltages no greater than 25 volts, power conversion efficiencies have been limited to no more than 1.times.10.sup.-5 w/w in organic devices. Organic electroluminescent devices using thick films (&gt;5.mu.), or single crystals, have been produced with power conversion efficiencies greater than 10.sup.-5 w/w. However, because of their greater thickness, the voltage required to drive such devices is quite high (.gtoreq.100 volts).
In order to reduce the driving voltage to no more than about 25 volts, thin-film electroluminescent devices are desirable, which as used herein means a device wherein the thickness of the active zones or layers, that is, the material between the electrodes, does not exceed about 1 micron. The thin film format has been particularly difficult to achieve in light of a pinholing problem. Pinholes are unacceptable because they short out the cell--see e.g., Dresner, RCA Review, Vol. 30, p 322ff (June 1969), and especially p. 326. To prevent formation of pinholes, a binder has been conventionally used in the coating formulations. Examples of such binders include addition polymers such as polystyrene, and condensation polymers such as polyesters. Although shorting of the cell may be avoided, the use of a binder is sometimes unsatisfactory. It requires the use of solvent coating manufacturing techniques, and the solvent of the one layer may also act as a solvent for the underlayer, thus preventing a sharp demarcation between layers. Although one could imagine a process of solvent-coating the one layer that needs a binder and then vapor depositing the layer(s) not needed a binder, a reverse sequence in which the luminescent layer is solvent-coated has not proven to be practical when the solvent affects the lower layer.
The cells described in commonly owned U.S. Application Serial No. 169,705, filed on July 17, 1980, by C. W. Tang entitled "Organic Electroluminescent Cell", now U.S. Pat. No. 4,356,429, are examples of markedly improved devices of the thin film format. Such cells have improved power conversion efficiencies by reason of reduced thickness of the luminescent zone, and of the use of an adjacent hole-injecting zone.
Although the cells of the aforesaid application have demonstrated the noted marked improvement over prior art cells, they have not achieved the levels of power conversion efficiencies that have been desired, that is, at least 9.times.10.sup.-5 w/w or higher when using a driving voltage no greater than 25 volts. The porphyrinic compounds in the hole-injecting layer are colored and thus tend to undesirably absorb some of the light that is emitted by the cell. Also, the porphyrinic compounds appear to interfere with the efficient radiative recombination of holes and electrons needed to efficiently generate light output.
Thus, what has been needed prior to this invention is an electroluminescent, hereinafter, "EL", device that has power conversion efficiencies improved by at least one order of magnitude, i.e., to at least 9.times.10.sup.-5 w/w, while maintaining the thin film format and reduced driving voltages.